1. Field of the Invention
The present invention relates to an antenna duplexer, a design method for the antenna duplexer, a production method for the antenna duplexer, a diplexer, a design method for the diplexer, a production method for the diplexer, and a communication apparatus.
2. Related Art of the Invention
In recent years, with the development of mobile communication, further miniaturization and improvement of performance of mobile communication apparatuses including a cellular phone has been requested.
An antenna duplexer 1100 used in such mobile communication apparatuses generally has a structure which comprises a transmission filter 1101, a reception filter 1102, phase-shift circuits 1103 and 1104 of adjusting phases of a transmission side and a reception side, and a junction point 1106 of connecting an antenna 1105, as shown in FIG. 18.
It has been considered desirable to design such a conventional antenna duplexer 1100 such that, in order to realize matching of impedances on the transmission side and the reception side, for example, in a transmission frequency band, an impedance at the time when the transmission filter 1101 side is viewed from the junction point 1106 is a value in the vicinity of 50Ω and an impedance at the time when the reception filter 1102 side is viewed from the junction point 1106 is close to infinity as much as possible. In a reception frequency band, according to the same idea, the antenna duplexer 1101 is designed such that the impedance at the time when the reception filter 1102 is viewed from the junction point 1106 is a value in the vicinity of 50Ω and an impedance at the time when the reception filter 1102 side is viewed from the junction point 1106 is close to infinity as much as possible (e.g., see Japanese Patent Application Laid-Open Nos. 2002-164710 (FIG. 3, etc.), 6-350305,6-350306, and 62-136105, the disclosures of which are incorporated herein by reference in their entireties). FIG. 19 is a schematic diagram showing, of the impedance characteristics of the antenna duplexer, an impedance on the reception filter 1102 side in the transmission frequency band (this impedance will be referred to as “impedance in a transmission attenuation band” in this specification) and an impedance on the transmission filter 1101 side in the reception frequency band (this impedance will be referred to as “impedance in a reception attenuation band” in this specification) on a Smith chart denoting the impedances with reference numeral 1201. These impedances 1201 in the transmission attenuation band and the reception attenuation band (this will be simply referred to as the impedance 1201 in an attenuation band) are indicated by a bold solid line on a circumference of the Smith chart in the figure.
Note that this impedance 1201 in the attenuation band has been considered ideal to set an impedance to infinity in the conventional design method. In addition, this bold solid line has a predetermined length because the attenuation bands for transmission and reception have widths of frequencies, respectively, and characteristics of both the attenuation bands overlap on the chart.
On the other hand, FIG. 20 shows a conventional example in which it is attempted to realize appropriateness of impedance matching from a viewpoint different from the above-mentioned conventional example.
In the conventional example shown in the figure, an antenna duplexer is designed such that a phase 1301a of an impedance 1301 in the transmission attenuation band of the reception filter 1102 and a phase 1302a of an impedance 1302 in the reception attenuation band of the transmission filter 1101 are vertically symmetrical with respect to a real axis 1303 of the Smith chart (e.g., see “A Miniaturized Dielectric Monoblock Duplexer for 1.9 GHz Band PCS Telephone System”, Takahiro Okada and two others, Nov. 7, 1996, Research Report of the Institute of Electronics, Information and Communication Engineers, Shingaku Giho CPM96-103, p. 55 to 60 (FIG. 7, Section 3.2.3, etc.), the disclosure of which is incorporated herein by reference in its entirety).
However, when the structure of the conventional example described with reference to FIG. 19 is adopted, it is difficult to realize sufficient matching in an entire range of a pass band because, for example, an impedance cannot be set to infinity actually and a frequency of an attenuation band has a predetermined width.
This point will be further described with reference to FIGS. 21(a) and 21(b).
Here, in FIG. 21(a), positions on a Smith chart of respective impedances at respective frequencies (fT1<fT0<fT2) of a lower limit, a center, and an upper limit of a pass band of a transmission filter are denoted by reference signs TfT1, TfT0 and TfT2. In addition, in FIG. 21(b), positions on a Smith charge of respective impedances at respective frequencies (fR1<fR0<fR2) of a lower limit, a center, and an upper limit of a pass band of a reception filter are denoted by reference signs RfR1, RfR0 and RfR2.
These positions are adjusted to be located in the vicinity of a center of the Smith chart. A phase-shift circuit is designed such that respective phases of the impedance (denoted by reference sign TfR0) at the frequency fR0 in the reception attenuation band 1401 of the transmission filter and the impedance (denoted by reference sign RfT0) at the frequency fT0 in the transmission attenuation band 1402 of the reception filter are 0 degree. However, the pass band and the attenuation band have widths of frequencies, and phases of the impedances at the respective frequencies are as described below. In the transmission filter of FIG. 21(a), a phase in a position denoted by reference sign TfR1 is 10.6 degrees, and a phase in a position denoted by reference sign TfR2 is −12.4 degrees. In the reception filter of FIG. 21(b), a phase in a position denoted by reference sign RfT1 is 22.8 degrees, and a phase in a position denoted by reference sign RfT2 is −13.7 degrees. All phases in the attenuation band 1401 and all phases in the attenuation band 1402 are not 0 degrees, that is, are not in a position of an open state.
In addition, concerning the conventional example shown in FIG. 20, it is difficult to design the antenna duplexer such that the positions of the impedances on the Smith chart are completely symmetrical in plural frequencies.
Further, in both FIGS. 19 and 20, since the antenna duplexer is designed taking into account only the impedances in the attenuation band, there is a doubt whether the antenna duplexer is optimized such that a signal loss is minimized in plural frequencies.